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Down and Dirty - The Arboretum’s Soil
John Neorr
About 15,000 years ago, if you were standing in the Arboretum, you would be very cold.
That’s because you would be standing on the Vashon glacier. 3,000 feet thick, this glacier
fully occupied the trough between the Olympic and Cascade Mountains and extended as
far south as Olympia. 4,000 years later, after this glacier retreated to Canada where it
belonged, it left behind the topology and parent soil material that you see today in the
Arboretum (and your backyard). Why is this important? Perhaps it is not, but learning
about the geologic history of Puget Sound can be a fun way to learn about soil and what
might or might not grow in it. This article describes the different types of soil that are
found in the Arboretum and also itemizes some of the native plants found in that soil.
Layering of Soil
The composition of soil is generally
categorized into 4 layers. The top layer (O) is
a thin layer of organic material that is
relatively undecomposed. In the forest, we
call this duff. Below this layer we have the
“A” layer – commonly called topsoil. Topsoil
is a layer of decomposed organic material and
a (typically) aerobic mix of sand, silt, and
clay and is where the roots of herbaceous
plants grow. Below the topsoil layer is the
subsoil (B) layer consisting of higher
concentrations of clay and other minerals
with some organic material. Roots of some
plants, especially trees, can penetrate this
layer of soil. The “C” layer of soil is the
parent material from which upper layers are
Soil Layers
derived. In the case of the arboretum, this
parent material is mostly glacial till (rocks deposited by a
glacier) or glacial outwash (gravels, sand, and silt deposited by glacier melt water).
The upper layers of soil seen today in undisturbed areas of the Arboretum are the result of
glacial materials weathering over thousands of years. Depending on climate and other
environmental conditions, it can take 1000 year to create a measly 2-4 cm of topsoil.
Arboretum Soil Types
The type and location of soils in the Arboretum was obtained from the US Department of
Agriculture Web Soil Survey (WSS) site: http://websoilsurvey.nrcs.usda.gov/app/. This
site is available to the public and, although somewhat painful to operate, can be used to
determine what kind of soil you have on your property (if you are not living on fill dirt).
The most predominate soil in the Arboretum is Everett (Ev). This is a fancy term for
“rocky” although some Everett soils have less rock than others. This soil originates from
glacial outwash which, as defined by the Arctic Geobotanical Atlas (AGA), is “gravel,
sand, and silt, commonly stratified, deposited by melt water as it flows from glacial ice.”
Everett soil can also have some volcanic ash in its upper portion. At least 90% of the soil
in the Arboretum is Everett. Most
is EvC which means that the land
is minimally sloped. The
Arboretum forest has some EvD
soil which means that the land is
sloped 15%-30%. Typical Everett
soil has an A layer thickness of 2
inches, B layer of 17 inches and a
C layer of 30 inches.
The two remaining types of soil
found in the Arboretum are Norma
(No) and Alderwood (AgD). The
Norma soil is found in the
southern end of the Smith
Mossman Azalea garden. Norma
soil consists of deep, poorly
drained soil formed in old
alluvium (sediment) from glacial
depressions. This seems to imply
that sometime in the distant past,
Arboretum map showing soil types
Lake Wilderness was larger than it
is today. The remaining soil type, Alderwood, is found in the southeast corner of the
Arboretum forest. Alderwood soil is a bit less acidic (pH 6.2 versus pH 5.6) than Everett
soil and is less rocky. My personal experience would suggest that Alderwood is more
likely to have a layer of hardpan than is Everett.
What grows where?
Everett and Alderwood soils tend to support the same species of native plants. Although
we do not have too much Alderwood soil, we find Douglas-fir, western hemlock, salal,
Oregon-grape, red huckleberry, and sword fern growing in both soils in the Arboretum.
Because Alderwood soil is somewhat loamier than Everett, it is more commonly used for
farmland and orchards. This bodes well for our miniature Garry oak woodland recently
planted in the Alderwood soil area. Some documentation also indicates that Alderwood
soil supports Pacific rhododendron, evergreen huckleberry, and Orange huckleberry
better than Everett soil. Conversely, the documentation states that ocean spray and
trailing blackberry prefer the Everett soil type. Not surprisingly, Norma soil is preferred
by wetland plants including thimbleberry, salmonberry, willow, skunkcabbage, sedges,
and rushes. In the natural areas of the Arboretum, over time Mother Nature has taken
care of “right plant, right place.” However for new plantings in the Arboretum, knowing
soil type will help guide our planting decisions. Likewise, you may want to consider
your soil type when thinking about planting native species in your backyard.